1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
2. Description of the Related Art
Conventionally, electrophotographic image forming apparatuses such as printers, photocopiers, and facsimiles have been widely used. At an image forming apparatus of this type, an image is transferred from an image forming part to paper conveyed from a paper feed part, and then the image is fixed to the paper by a fixing device. The fixing device includes a heating roller with a plurality of heaters therein and a pressure roller to be brought into contact with the heating roller under pressure. The heaters are halogen lamps, for example.
In recent years, there have been increasing demands for energy saving in image forming apparatuses. Thus, to achieve energy saving in image forming apparatuses, the fixing device has been thinned at a core constituting the fixing roller to reduce the heat capacity of the fixing part. However, when the fixing roller has the thinned core as described above, the heat escapes from support members supporting the both end portions of the fixing roller, and the temperature of the fixing roller tends to be lower at the end portions than at the central portion thereof. As a result, temperature variations occur on the surface of the fixing roller to increase temperature ripples on the fixing roller.
To cope with these problems, for example, JP 2007-310254 A discloses an image forming apparatus that sets the power ratio of a sub heater upon energization corresponding to the power ratio of a main heater, according to the temperature of the end portions of the fixing roller detected by a second temperature detecting unit. In addition, JP 2013-148721 A discloses a fixing device that, in the case of feeding small-sized paper, causes a sub heater to start PID temperature control, and decides the initial lighting ratio at the start of the PID temperature control by the sub heater according to the difference between the temperature detected by each temperature detecting unit at the start of the control and the target paper-feed temperature for printing jobs.
However, the fixing devices described in JP 2007-310254 A and JP 2013-148721 A have problems described below. Specifically, at the image forming apparatus disclosed in JP 2007-310254 A, both the main heater and the sub heater are subjected to lighting control in half-wave on/off patterns. However, the lighting control is not conducted in the on/off patterns at the optimum duty ratios, and thus the temperature of the end portions of the fixing roller does not increase, which brings about unevenness in the temperature of the entire fixing roller. In addition, the simultaneous duty controls of the main heater and the sub heater lead to complicated operations. At the fixing device disclosed in JP 2013-148721 A, for feeding of small-sized paper, the main heater is turned off and the sub heater conducts the PID temperature control, which causes problems such as temperature non-uniformity of the fixing roller and occurrence of temperature ripples on the fixing roller.